Dams and levees are built to contain waters and earthen levees are generally constructed using soils extracted nearby and vary in permeability and strength. Failure may be due to seepage, piping-dislocation, and subtle movement due to inadequate design, poor maintenance or excessive hydraulic loads.
Levees and dams are built with a factor of safety based on their size, expected height and rate of flow (flow nets) of water through the permeable soils of which they are constructed. The high flow line is a function of the dimensions of the levee and is part of the safety design factor. Detection in movement of the flow line may indicate seepage in the levee or dam. Prior warning of failure is usually unavailable until sand boils, major deformation, or both, are observed. It would be advantageous to identify minor movement, measurement of vibrations, or both in these structures before catastrophic failure. Several conventional methods are used to detect weakness. The first is to take many soil samples from “suspect” areas and ascertain sand content. This is expensive, labor intensive, and marginally effective. Other methods involving visual inspection during both dry and wet weather periods are less than satisfactory. Further, these methods do not provide continuous monitoring. Technical methods such as the use of ground-penetrating radar are limited by the small size of the sand channel and moisture content within the outer surface being surveyed. Externally applied electromagnetic fields (EMF) have low resolution, inadequate depth of penetration and produce target ambiguities and concomitant difficulty in data interpretation.
Accordingly there is a need for monitoring these structures, either continuously or at pre-specified intervals, to provide real time or near real lime status on structural condition, often established by comparing change in both soil saturation levels and soil displacement along the structure.